Background: Differential diagnosis between polymyalgia rheumatica (PMR) and seronegative elderly-onset rheumatoid arthritis (SEORA) is not easy, to the point that in the past they were considered the same entity. In these patients, sleep disorders have been scarcely assessed, and considered as expression of mood disorders such as depression and anxiety. Methods: In 38 Caucasian elderly patients (median age: 73.9 ± 8.06 years) consecutively referred to two outpatient clinics from January to May 2018 with diagnosis of PMR and SEORA, sleep impairment was assessed using the Medical Outcomes Study-Sleep scale (MOS-SS). Depression and anxiety were assessed using the Neuropsychiatric Inventory (NPI) score, with point 0 for absent and point 3 for severe. Comorbidities were assessed using the Cumulative Illness Rating Scale (CIRS). Patients taking medications used to treat sleep disturbance or that could favor sleep disturbances were excluded. The study was approved by the local ethics committee and carried out in accordance with the Helsinki Declaration, revised 2013. Every patient signed an informed consent form at the time of the first visit. Results: MOS-SS total point in PMR patients was significantly higher than in SEORA patients (47.60 ± 8.4 vs 28.26 ± 12.4; P = 0.000). After six-month therapy with prednisone (12.5–15 mg/day, followed after 4 weeks by gradual tapering), MOS-SS total point improved in the two groups of patients, with no significant difference (17.0 ± 6.2 vs 17.8 ± 4.2; P = 0.644). No correlation was found between MOS-SS and comorbidities, and between MOS-SS, anxiety or depression. Conclusions: Our data suggest that the assessment of sleep impairment could be very useful in the differential diagnosis between PMR and SEORA. Up today, the reasons why patients with PMR have—at the time of diagnosis—a sleep impairment higher than SEORA are speculative. Further ad hoc complementary studies in multicenter cohorts are needed.